There are two major Stage 2 vapor recovery systems in use: the balanced system and the vacuum assist system.
The balanced system uses the positive pressure created in a gasoline tank during fueling, which forces the gasoline vapors from the tank through the vapor recovery nozzle and a special gasoline/vapor hose back into the service station storage tank.
The vacuum assist system, in addition to a vapor recovery nozzle and hose, requires a vacuum pump which provides a vacuum assist for transporting the vapors from the automobile tank back into the storage tank. Because of the additional equipment, the vacuum assist system is more costly to install and maintain. For that reason, the balanced system has been preferred by the industry.
The existing vapor recovery nozzles for either system are difficult to handle, difficult to insert and difficult to seal with automobile fill pipes. Both systems are prone to gasoline splash-back and spillage and are hated by both service station owners and motorists.
In addition, the rubber bellow/boot that is used to make a seal on the existing vapor recovery nozzles are prone to cuts. The cuts nullify the efficiency of the entire vapor recovery system. When detected by EPA inspectors, cuts in boots result in large fines to the service station owners.
One of the requirements for any type of vapor recovery system is that liquid must not be aspirated into the vapor return conduit from the vehicle tank or fill pipe or from other parts of the nozzle, otherwise customers would be charged for fluid which was aspirated back to the storage tank after having passed through the dispenser's meter.
To achieve the required efficiency of the vapor recovery system, the vacuum generated at the pump must be maintained at a particular level. If the vacuum is too high, fluid will be aspirated back into the vapor recovery line and the customer will be charged for fluid he has not received.
If the vacuum is not sufficient, or is too low, the required efficiency of the vapor recovery system will not be achieved, and an excessive amount of vapor will be released into the atmosphere.
In the vacuum assist vapor recovery systems currently in use, a centrifugal vacuum pump is positioned in the storage tank area. Vacuum is generated in the storage tank and must be transmitted through the piping and the entire length of the flexible hose that connects the dispenser with the vapor recovery nozzle.
This arrangement creates fluctuations of the vacuum level at the nozzle/filler neck connection due to variations in the number of dispensers used at the same time and changes in the hose restrictions as functions of vehicle-dispenser orientations.
Another arrangement described in U.S. Pat. No. 3,826,291 employs a positive displacement pump, such as a vane pump, at each dispenser. It must also maintain the sufficient vacuum through the entire length of the flexible hose to the nozzle. The initial high cost, as well as the inherent high wear and maintenance cost, make that solution an unattractive choice for service station owners.
A need exists for an improved system for vapor recovery in gasoline dispensing.